1. Field of the Invention
This invention relates to improvements in: an oxide evaporation material used when a transparent conducting film and a high-refractive-index transparent film are formed by any of various vacuum deposition methods such as electron beam deposition, ion plating, and high-density plasma-assist evaporation; and a high-refractive-index transparent film formed using the oxide evaporation material.
2. Description of the Related Art
A transparent conducting film has a high conductivity and a high transmittance in the visible region. By taking advantage of these characteristics, the transparent conducting film is utilized as an electrode or the like of solar cells, liquid crystal display elements, and various other light receiving elements. Furthermore, by taking advantage of the reflection and absorption characteristics in the near-infrared region, the transparent conducting film is utilized also as: a heat-ray reflection film used for window glasses of automobiles, architectures, and the like; a variety of antistatic films; and an anti-fogging transparent heater for refrigerated showcases or the like.
Generally, the widely used transparent conducting films are formed of: tin oxide (SnO2) containing antimony or fluorine as a dopant; zinc oxide (ZnO) containing aluminum, gallium, indium, or tin as a dopant; indium oxide (I2O3) containing tin, tungsten, or titanium as a dopant; and the like. Particularly, an indium oxide film containing tin as a dopant, i.e., an In2O3—Sn film is referred to as an indium tin oxide (ITO) film, and is industrially widely used to date because a low-resistance transparent conducting film is easily obtained.
As to a method for forming such transparent conducting films, generally used are vacuum deposition methods, sputtering methods, and methods involving application of a coating for forming a transparent conducting layer. Above all, the vacuum deposition methods and the sputtering methods are effective methods for a case where a material having a low vapor pressure is used or where precise film thickness control is required. Moreover, these methods are very simple in operation and thus industrially useful. As the vacuum deposition methods are compared with the sputtering methods, the vacuum deposition methods are capable of forming a film at a faster rate and thus superior in productivity.
In the vacuum deposition methods, generally, a solid or liquid evaporation source is heated in a vacuum of approximately 10−3 to 10−2 Pa and temporarily decomposed to gas molecules or atoms which are then condensed on the surface of a substrate as a thin film again. General methods for heating an evaporation source are a resistance heating method (RH method) or an electron-beam heating method (EB method, electron beam deposition). Additionally, methods utilizing laser light, high-frequency induction heating, and the like are also available. Furthermore, flash evaporation, arc plasma deposition, reactive evaporation method, and the like are also known. The vacuum deposition method includes these methods.
The electron beam deposition has been historically frequently utilized for depositing an oxide film such as ITO. Specifically, an ITO oxide evaporation material (may also be called an ITO tablet or an ITO pellet) is used as the evaporation source, and an O2 gas serving as the reactive gas is introduced into a film-formation chamber (chamber). Thermal electrons jumped off from a thermal-electron generating filament (mainly a W wire) are accelerated by an electric field and radiated to the ITO oxide evaporation material. The oxide evaporation material is locally heated at the radiated area thereof, and evaporated and deposited to a substrate. Meanwhile, activated reactive evaporation (ARE method) is also a useful method for ITO film formation. In this method, a plasma is generated using a thermal electron emitter or RF discharge, and an evaporation material and a reactive gas (O2 gas, or the like) are activated by this plasma, thereby forming a low-resistance film on a low-temperature substrate. Furthermore, high-density plasma-assist evaporation (HDPE method) using a plasma gun also has been revealed to be an effective method for ITO film formation, and begun to be industrially widely used recently [see “Vacuum,” Vol. 44, No. 4, 2001, pp. 435-439 (hereinafter, “Non-Patent Document 1”)]. This method utilizes an arc discharge using a plasma generator (plasma gun). The arc discharge is maintained between a cathode inside the plasma gun and a crucible (anode) of an evaporation source. Electrons emitted from the cathode are guided by a magnetic field, concentrated and radiated to a local area of an ITO oxide evaporation material put in the crucible. An evaporant is generated from the area that is locally heated by the radiation of the electron beams, and deposited to a substrate. The vaporized evaporant and an introduced O2 gas are activated in this plasma, so that an ITO film having favorable electrical characteristics can be formed. Meanwhile, as another classification of these various vacuum deposition methods, those involving ionization of an evaporation material and a reactive gas are collectively referred to as ion plating (IP method). Ion plating is effective as a method to obtain an ITO film having a low resistance and a high transmittance [see “Technology of transparent conductive film,” Ohrmsha, Ltd., 1999, pp. 205-211 (hereinafter, “Non-Patent Document 2”)].
In any type of solar cell using the transparent conducting film, the transparent conducting film is essential for an electrode on the front side from which light enters the cell. As the transparent conducting film, the aforementioned ITO film, a zinc oxide film doped with aluminum (AZO), or a zinc oxide film doped with gallium (GZO) has been conventionally utilized. These transparent conducting films are required to have such characteristics as a low resistance and a high transmittance of visible light. As methods for forming these transparent conducting films, the above-described vacuum deposition methods such as ion plating and high-density plasma-assist evaporation are used.
Meanwhile, the above-described thin film containing indium oxide, tin oxide, or zinc oxide as the main component is utilized also as an optical film. Such a thin film is a high-refractive-index material having a refractive index of 1.9 to 2.1 in the visible region, and is capable of demonstrating an optical interference effect when formed into a laminate in combination with a low-refractive-index film such as a silicon oxide film and metal fluoride film which have a refractive index of 1.3 to 1.5 in the visible region. Specifically, when the laminate is formed under the precise control of the thickness of each film, the laminate can have a reflection preventing effect or a reflection enhancing effect in a particular wavelength region. For this usage, a high-refractive-index film having a higher refractive index is more useful since a stronger interference effect can be easily obtained.
Japanese Patent Laid-open Application No. 2005-242264 (hereinafter, “Patent Document 1”) states that an indium oxide film containing cerium has a higher refractive index than the aforementioned tin oxide film, zinc oxide film, and the like. Patent Document 1 proposes an example of using the film as an optical film. Furthermore, Japanese Patent No. 3445891 and Japanese Patent Laid-open Application No. 2005-290458 (hereinafter, respectively “Patent Documents 2 and 3”) each propose techniques related to: a sputtering target material made of indium oxide containing cerium (In—Ce—O); and a transparent conducting film obtained by sputtering the sputtering target material. Moreover, Patent Document 2 states that since the indium oxide-based transparent conducting film containing cerium proposed therein poorly reacts with Ag, a transparent conducting film having a high transmittance and excellent heat resistance can be formed when the indium oxide-based transparent conducting film is stacked on a Ag-based ultra-thin film. Patent Document 3 states that a film having excellent etching characteristics is obtained.
When a thin film such as a transparent conducting film is formed by vacuum deposition methods such as electron beam deposition, ion plating, and high-density plasma-assist evaporation, a sintered body small in size (for example, having a diameter of approximately 10 to 50 mm, a height of approximately 10 to 50 mm, and a cylindrical shape) is used as an oxide evaporation material in the vacuum deposition methods. This limits the amount of film that can be formed from a single oxide evaporation material. Moreover, when the remaining amount of oxide evaporation material is decreased as the consumed amount is increased, the following procedure has to be performed: terminating the film formation; introducing air into the film-formation chamber in the vacuum state for replacement with a fresh oxide evaporation material yet to be used; and evacuating the film-formation chamber again. This consequently lowers the productivity.
Essential techniques adopted in mass production of transparent conducting films by the vacuum deposition methods such as electron beam deposition, ion plating, and high-density plasma-assist evaporation include a method of continuously supplying the oxide evaporation materials. Non-Patent Document 1 describes an example of such a continuous supply method. In the continuous supply method, cylindrical oxide evaporation materials are housed in series inside a cylindrical hearth, and are sequentially pushed out and continuously supplied while the height of the sublimation surface is kept the same. The continuous supply method of an oxide evaporation material enables mass production of transparent conducting films by the vacuum deposition methods.
An indium oxide film containing cerium is normally formed by sputtering methods, as proposed in Patent Documents 2 and 3. However, in recent years, various vacuum deposition methods that are advantageous in terms of productivity are highly demanded for the film formation.
However, there are few technique related to an oxide evaporation material for stably forming an indium oxide film containing cerium by vacuum deposition methods. A technique for manufacturing a sintered body of a sputtering target has been adopted so far to manufacture an oxide evaporation material.
In a case of forming a high-refractive-index transparent film having a low resistance and a high transmittance by any of various vacuum deposition methods such as electron beam deposition, ion plating, and high-density plasma-assist evaporation, using an oxide evaporation material manufactured by the technique adopted so far, a large amount of oxygen gas needs to be introduced into a film-formation vacuum chamber during the film formation. This brings about problems mainly described below.
First, the transparent film and the oxide evaporation material greatly differ in composition from each other, making it difficult to design the composition of the transparent film. This is because, generally, when a larger amount of oxygen is introduced into a film-formation vacuum chamber, the difference in composition between a transparent film and an oxide evaporation material is likely to increase. In the mass production process of films, the amount of oxygen in a film-formation vacuum chamber also tends to vary. Due to the variation in the oxygen amount, the compositions of the films are likely to differ from one another, resulting in the variation of the film characteristics.
Moreover, when the oxygen amount is increased, film formation by reactive evaporation using an oxygen gas causes problems that not only does the film density decrease, but also the adhesive force of the film to the substrate weakens, for example. These problems occur for the following reason. Specifically, when evaporated metal oxide is oxidized before reaching the substrate, the energy is lost. Thus, an increase in the oxidation ratio makes it difficult to obtain a dense film strongly adhering to the substrate.
Furthermore, suppose a case where a high-refractive-index transparent film is formed on a substrate covered with a metal film or an organic film having a surface that can be oxidized easily. In this case, if a large amount of oxygen gas is introduced into a film-formation vacuum chamber, the substrate surface is oxidized before the film formation. This hinders fabrication of a high-performance device. This tendency becomes more noticeable as the temperature of the substrate during the film formation is higher. For example, assume a case where an optical thin film laminate having an interference effect by stacking an ultra-thin metal film such as Ag and a high-refractive-index transparent film. In this case, if the high-refractive-index transparent film is formed on the surface of the ultra-thin metal film with a large amount of oxygen introduced, the ultra-thin metal film is oxidized, hindering formation of a favorable optical thin film laminate.